The present invention relates to power multiplication of microwave signals and more specifically to pulse compression of high power microwave pulses with a plasma filled waveguide.
Use of a frequency chirped (frequency changing in time) pulse in conjunction with a passive dispersive delay element to compress microwave pulses was invented in the World War II era. If the group velocity of an electromagnetic wave increases with frequency in a dispersive device, such as in a waveguide above cutoff, then the tail end of a microwave pulse can catch up to the leading edge, providing the pulse frequency is increased throughout the pulse duration. For many radar applications, the dispersion and compression may be done either in the transmit or receive phase. The chirped-pulse, dispersive-delay-element, pulse-compression scheme, as well as use of a microwave waveguide as the dispersive element was disclosed in U.S. Pat. No. 2,624,876, and the use of passive lumped electrical circuitry in U.S. Pat. No. 2,678,997. The same basic compression idea has been applied to acoustic pulse compression (British Patent No. 604,429) and laser pulse compression (E. A. McLean, R. H. Lehmberg, J. M. McMahon, B. H. Ripin, J. A. Stamper, and S. E. Bodner, "Shadowgraph and Interferometry Experiments with a Ten-picosecond Light Source at Various Wavelengths," SPIE Vol. 97 International Conference on High Speed Photography (1977)).
The ideas of using plasma filled devices as passive microwave components, L. Goldstein, IRE Trans. MTT-6, 19 (1958), and the use of very short pulse propagation as a density plasma diagnostic, H. Schmitt, IEEE Trans. Nucl. Sci. NS-11, 125 (1964), H. Schmitt, IEEE Trans. MTT-13, 472 (1965), are quite old. Goldstein utilized the difference in phase velocity of the two principal waves along the magnetic field in a plasma to produce Faraday rotators and phase shifters. Also, he suggested an isolator based on the difference in damping rate between the right and left hand circularly polarized waves. The problem with these devices, however, was that they were based on gas discharges and the fractional ionization was very low, typically about 10.sup.-5. Thus, the physics turned out to be dominated in great measure by electron neutral collisions. For instance, Goldstein has shown that at microwave powers as low as six milliwatts, the resonant heating of the electrons due to electron-neutral collisions gives a large shot-to-shot variation in the characteristics of the device.
All of the above-identified schemes for pulse compression are not able to attain pulse compression of very high power microwaves i.e., achieving power multiplication of 10-100 times a gigawatt of input energy.